THE CAMBRIAN EXPLOSION
The geologic timescale is marked by significantgeologic and biological events, including the origin of Earth about 4.6 billionyears ago, the origin of life about 3.5 billion years ago, the origin ofeukaryotic life-forms (living things that have cells with true nuclei) about 1.5billion years ago, and the origin of animals about 0.6 billion years ago. Thelast event marks the beginning of the Cambrian period. Animals originatedrelatively late in the history of Earth in only the last 10 percent of Earth’shistory. During a geologically brief 100-million-year period, all modern animalgroups (along with other animals that are now extinct) evolved. This rapidorigin and diversification of animals is often referred to as “the Cambrianexplosion.”
Scientists have asked important questions about thisexplosion for more than a century. Why did it occur so late in the history ofEarth? The origin of multicellular forms of life seems a relatively simple stepcompared to the origin of life itself. Why does the fossil record not documentthe series of evolutionary changes during the evolution of animals? Why didanimal life evolve so quickly? Paleontologists continue to search the fossilrecord for answers to these questions.
One interpretation regarding theabsence of fossils during this important 100-million-year period is that earlyanimals were soft bodied and simply did not fossilize. Fossilization ofsoft-bodied animals is less likely than fossilization of hard-bodied animals,but it does occur. Conditions that promote fossilization of soft-bodied animalsinclude very rapid covering by sediments that create an environment thatdiscourages decomposition. In fact, fossil beds containing soft-bodied animalshave been known for many years.
The Ediacara fossil formation, whichcontains the oldest known animal fossils, consists exclusively of soft-bodiedforms. Although named after a site in Australia, the Ediacara formation isworldwide in distribution and dates to Precambrian times. This700-million-year-old formation gives few clues to the origins of modern animals,however, because paleontologists believe it represents an evolutionaryexperiment that failed. It contains no ancestors of modern animalgroups.
A slightly younger fossil formation containing animal remains isthe Tommotian formation, named after a locale in Russia. It dates to the veryearly Cambrian period, and it also contains only soft-bodied forms. At one time,the animals present in these fossil beds were assigned to various modern animalgroups, but most paleontologists now agree that all Tommotian fossils representunique body forms that arose in the early Cambrian period and disappeared beforethe end of the period, leaving no descendants in modern animal groups.
Athird fossil formation containing both soft-bodied and hard-bodied animalsprovides evidence of the result of the Cambrian explosion. This fossilformation, called the Burgess Shale, is in Yoho National Park in the CanadianRocky Mountains of British Columbia. Shortly after the Cambrian explosion, mudslides rapidly buried thousands of marine animals under conditions that favoredfossilization. These fossil beds provide evidence of about 32 modern animalgroups, plus about 20 other animal body forms that are so different from anymodern animals that they cannot be assigned to any one of the modern groups.These unassignable animals include a large swimming predator called Anomalocarisand a soft-bodied animal called Wiwaxia, which ate detritus or algae. TheBurgess Shale formation also has fossils of many extinct representatives ofmodern animal groups. For example, a well-known Burgess Shale animal calledSidneyia is a representative of a previously unknown group of arthropods (acategory of animals that includes insects, spiders, mites, andcrabs).
Fossil formations like the Burgess Shale show that evolutioncannot always be thought of as a slow progression. The Cambrian explosioninvolved rapid evolutionary diversification, followed by the extinction of manyunique animals. Why was this evolution so rapid? No one really knows. Manyzoologists believe that it was because so many ecological niches were availablewith virtually no competition from existing species. Will zoologists ever knowthe evolutionary sequences in the Cambrian explosion? Perhaps another ancientfossil bed of soft-bodied animals from 600-million-year-old seas is awaitingdiscovery.
Paragraph1: The geologic timescale is marked bysignificant geologic and biological events, including the origin of Earth about4.6 billion years ago, the origin of life about 3.5 billion years ago, theorigin of eukaryotic life-forms (living things that have cells with true nuclei)about 1.5 billion years ago, and the origin of animals about 0.6 billion yearsago. The last event marks the beginning of the Cambrian period. Animalsoriginated relatively late in the history of Earth in only the last 10 percentof Earth’s history. During a geologically brief 100-million-year period, allmodern animal groups (along with other animals that are now extinct) evolved.This rapid origin and diversification of animals is often referred to as “theCambrian explosion.”
1. The word significant in the passage is closest inmeaning to
A numerous
B important
C unexplained
D sudden
2. Theword relatively in the passage is closest in meaningto
A surprisingly
B collectively
C comparatively
D characteristically
3.The word diversification in the passage is closest in meaning to
A emergenceof many varieties
B steady decline in number
C gradual increase in bodysize
D sudden disappearance
Paragraph2: Scientists have asked importantquestions about this explosion for more than a century. Why did it occur so latein the history of Earth? The origin of multicellular forms of life seems arelatively simple step compared to the origin of life itself. Why does thefossil record not document the series of evolutionary changes during theevolution of animals? Why did animal life evolve so quickly? Paleontologistscontinue to search the fossil record for answers to these questions.
4.The period discussed in the passage is referred to as an explosion becauseit
A occurred 0.6 billion years ago, late in Earth’s history
B wascharacterized by the unusually fast evolution of many new life-forms.
C Wascharacterized by widespread animal extinction
D Was characterized by violentvolcanic eruptions
5. According to Paragraph2, which of the following isNOT a question that paleontologists asked about the Cambrian explosion?
A Whywas the origin of life a simple step in Earth’s history?
B Why did it take solong for multicellular organisms to develop?
C Why did animal life evolve sorapidly?
D Why does the fossil record lack evidence of animal evolution duringthat time?
6. Which of the following best describes the relationshipbetween Paragraph2 and Paragraph3?
A Paragraph2 puts forward severalscientific claims, one of which is rejected in Paragraph3
B Paragraph2 posesseveral questions, and Paragraph3offers a possible answer to one ofthem
C Paragraph2 presents outdated traditional views, whileParagraph3presents the current scientific conclusions.
D Paragraph2 introducesa generalization that is illustrated by specific examples inParagraph3
Paragraph3: One interpretation regarding the absence offossils during this important 100-million-year period is that early animals weresoft bodied and simply did not fossilize. Fossilization of soft-bodied animalsis less likely than fossilization of hard-bodied animals, but it does occur.Conditions that promote fossilization of soft-bodied animals include very rapidcovering by sediments that create an environment that discourages decomposition.In fact, fossil beds containing soft-bodied animals have been known for manyyears.
7. The word promote in the passage is closest in meaningto
A complicate
B prevent
C encourage
D affect
Paragraph4:The Ediacara fossil formation, which contains the oldest known animal fossils,consists exclusively of soft-bodied forms. Although named after a site inAustralia, the Ediacara formation is worldwide in distribution and dates toPrecambrian times. This 700-million-year-old formation gives few clues to theorigins of modern animals, however, because paleontologists believe itrepresents an evolutionary experiment that failed. It contains no ancestors ofmodern animal groups.
8. Which of the following is NOT mentioned inParagraph4 as being true of the Ediacara formation?
A It contains fossils thatdate back to the Precambrian period.
B It contains only soft-bodied animalfossils.
C It is located on a single site in Australia.
D It does notcontain any fossils of the ancestors of modern animals.
Paragraph5: Aslightly younger fossil formation containing animal remains is the Tommotianformation, named after a locale in Russia. It dates to the very early Cambrianperiod, and it also contains only soft-bodied forms. At one time, the animalspresent in these fossil beds were assigned to various modern animal groups, butmost paleontologists now agree that all Tommotian fossils represent unique bodyforms that arose in the early Cambrian period and disappeared before the end ofthe period, leaving no descendants in modern animal groups.
9. Which ofthe sentences below best expresses the essential information in the highlightedsentence in the passage? Incorrect choices change the meaning in important waysor leave out essential information.
A The animals found in the Tommotianfossil bed were once thought to belong to a variety of modern animal groups, butnow they are thought to have descended from a single group.
B Animals in theTommotian fossil beds were initially assigned to modern animal groups but arenow thought to belong to groups that emerged and died out during the Cambrianperiod.
C Though at first they thought otherwise, paleontologists now agreethat the animals in the Tommotian have body forms from which modern animals havedescended.
D It is unclear whether the Tommotian fossils from the earlyCambrian period represent unique body forms or whether they should be assignedto various modern animal groups.
Paragraph6: A third fossil formationcontaining both soft-bodied and hard-bodied animals provides evidence of theresult of the Cambrian explosion. This fossil formation, called the BurgessShale, is in Yoho National Park in the Canadian Rocky Mountains of BritishColumbia. Shortly after the Cambrian explosion, mud slides rapidly buriedthousands of marine animals under conditions that favored fossilization. Thesefossil beds provide evidence of about 32 modern animal groups, plus about 20other animal body forms that are so different from any modern animals that theycannot be assigned to any one of the modern groups. These unassignable animalsinclude a large swimming predator called Anomalocaris and a soft-bodied animalcalled Wiwaxia, which ate detritus or algae. The Burgess Shale formation alsohas fossils of many extinct representatives of modern animal groups. Forexample, a well-known Burgess Shale animal called Sidneyia is a representativeof a previously unknown group of arthropods (a category of animals that includesinsects, spiders, mites, and crabs).
10. Why does the author mentionAnomalocans and Wiwaxia?
A To contrast predators with animals that eat plantssuch as algae
B To question the effects of rapid mud slides onfossilization
C To suggest that much is still unknown about animals found inthe Burgess Shale
D To provide examples of fossils that cannot be assigned toa modern animal group
11. Sidneyia is an example ofa relative ofAnomalocaris and Wiwaxia
A a previously unknown Burgess Shale animal
B anextinct member of a currently existing category of animals
C an animal thatcannot be assigned to any modern animal group
Paragraph7: Fossilformations like the Burgess Shale show that evolution cannot always be thoughtof as a slow progression. The Cambrian explosion involved rapid evolutionarydiversification, followed by the extinction of many unique animals. Why was thisevolution so rapid? No one really knows. Many zoologists believe that it wasbecause so many ecological niches were available with virtually no competitionfrom existing species. Will zoologists ever know the evolutionary sequences inthe Cambrian explosion? Perhaps another ancient fossil bed of soft-bodiedanimals from 600-million-year-old seas is awaiting discovery.
12. Whatcan be inferred from Paragraph7 about why the Cambrian explosion is sounusual?
A It generated new ecological niches through the extinction of manyunique animals.
B It was a period of rapid evolution, and evolution is oftenthought of as a slow process.
C It is a period whose evolutionary sequencesare clearly marked.
D It generated a very large number of ancient fossil bedscontaining soft-bodied animals.
Paragraph3: One interpretation regardingthe absence of fossils during this important 100-million-year period is thatearly animals were soft bodied and simply did not fossilize. ■ Fossilization ofsoft-bodied animals is less likely than fossilization of hard-bodied animals,but it does occur. ■ Conditions that promote fossilization of soft-bodiedanimals include very rapid covering by sediments that create an environment thatdiscourages decomposition.■ In fact, fossil beds containing soft-bodied animalshave been known for many years. ■
13. Look at the four squares [■] thatindicate where the following sentence could be added to the passage.
A It isrelatively rare because the fossilization of soft-bodied animals requires aspecial environment.
B Where could the sentence bestfit?
14. Directions: An introductory sentence for a briefsummary of the passage is provided below. Complete the summary by selecting theTHREE answer choices that express the most important ideas in the passage. Someanswer choices do not belong in the summary because they express ideas that arenot presented in the passage or are minor ideas in the passage. This question isworth 2 points.
The term “Cambrian explosion” refers to thegeologically brief period during which all modern animal groupsevolved.
AnswerChoices
1. Little is known about the stages of evolution during the Cambrianperiod, in part because early animals were soft bodied and could fossilize onlyunder particular conditions.
2. While animal fossils from before the Cambrianexplosion have no modern descendants, many animals that evolved during theCambrian explosion can be assigned to modern groups.
3. The Cambrian periodis significant because it marks the emergence of eukaryotic life-forms organisms that have cells with true nuclei
4. The Ediacara fossil formationprovides the most information about the Cambrian explosion, while the earlier,Tommotian and Burgess Shale formations give clues about Precambrianevolution.
5. Zoologists are awaiting the discovery of a 600-million-year-oldfossil formation in order to be able to form a theory of how animal evolutionprogressed.
6. Although the reasons for the rapid evolution of animals duringthe Cambrian period are not known, one proposed explanation is an abundance ofniches with a lack of competitors.
参考答案
1.B
2.C
3.A
4.B
5.A
6.B
7.C
8.C
9.B
10.D
11.C
12.B
13.B
14. 1 2 6